1. Field of the Invention
This invention relates generally to megasonic processing apparatus and associated methods involving one or more piezoelectric transducers operating in thickness mode at megasonic frequencies of at least 300 KHz or higher, and relates more particularly to improving performance by sweeping the frequency of a driving signal throughout a predetermined or programmable frequency range that spans the resonant frequencies of all the transducers.
2. Description of the Relevant Art
Megasonic processing involves generating and using high frequency energy at frequencies above 300 KHz. Many megasonic systems operate at frequencies at or near 1,000 KHz, or one megahertz. Although 1 MHz is the consensus, preferred frequency for many applications, the frequency range goes much higher, with frequencies as high as 10 MHz. Typical uses for megasonic systems include cleaning delicate objects, such as semiconductor wafers and disc drive media. Such a megasonic cleaning process involves placing the objects to be cleaned in a fluid-filled tank, and applying vibrational energy at megasonic frequencies to a radiating surface or surfaces of the tank. One or more piezoelectric transducers are used to generate the vibrational energy. A generator supplies an alternating current driving signal at the resonant frequency of the transducers. Megasonic transducers operate in thickness mode, where a piezoelectric element is excited by an alternating current driving signal that causes alternating expansion and contraction of the transducer, primarily expanding and contracting the thickness of the transducer. A piezoelectric transducer having a thickness of 0.080 inches has a fundamental, thickness mode, resonant frequency of 1,000 KHz.
Megasonic processing has some similarities with ultrasonic processing, which involves lower fundamental frequencies, typically from about 25 KHz to about 192 KHz. Ultrasonic transducers are typically mass-balanced, with inert masses on either side of a piezoelectric element, and have a significant radial component of movement at right angles to the thickness component. One common construction of an ultrasonic transducer is to stack several layers of ring-shaped piezoelectric elements between two masses, and to hold the assembly together with an axial compression bolt. Ultrasonic cleaning is based on cavitation, which is the formation and collapse of bubbles in the fluid.
At the frequencies used for megasonic cleaning, significant cavitation does not occur, so the cleaning action is based on another mechanism known as micro-streaming, which is a general flow of detached particles flowing away from the megasonic transducers. This flow consists of planar waves originating at the surface to which the transducers are mounted. The planar nature of these micro-streams affects the distribution of megasonic energy throughout the tank. One way to improve the distribution is to cover a high percentage of the surface area of the tank with transducers. Another but less efficient way is to oscillate or move the parts to be processed throughout the tank so that all surfaces are exposed to sufficiently high megasonic energy.
It is known that radical-mode ultrasonic activity in a cleaning tank may benefit from a process of sweeping or varying the frequency of the driving signal. However, there has been an industry-wide belief that you cannot sweep megasonic frequencies because the sound waves are too small and weak for any benefit from sweeping. In addition, it has been thought that there would be no benefit from sweeping megasonic frequencies because of the thickness mode transducers and resultant planar nature of megasonic vibrations and due to the different cleaning mechanisms at work as compared to ultrasonics.